Melt-blends of polymers are well-known. Cycloolefin containing polymers or COPs have been added to polypropylene resins as seen for example in U.S. Pat. No. 6,068,936 which discloses a polyolefin film which includes at least one layer containing polyolefin and cycloolefin polymer, where the cycloolefin polymer is amorphous and has a mean molecular weight in the range from 200 to 100,000. The blend is further characterized in that the mean molecular weight of the cycloolefin polymer is at most 50% of the mean molecular weight of the polyolefin, and the cycloolefin polymer is a homopolymer or a copolymer containing at most 20% by weight of comonomer. COPs and polyethylene blends are disclosed in U.S. Pat. No. 6,696,542 wherein such blends comprise an ethylene based polymer having a glass transition temperature lower than or equal to 20° C. and a cycloolefin polymer.
U.S. Pat. No. 6,670,423 discloses blends comprising a polar derivative of an ethylene based polymer and a cycloolefin polymer. Additionally, U.S. Pat. No. 6,329,456 discloses an ethylene copolymer composition which comprises an ethylene/α-olefin copolymer and high-pressure radical process low-density polyethylene. PCT 2003/025395 discloses a multi-layer heat film having at least three layers wherein the film has optical clarity, stiffness and low shrinkage in the cross direction. The inner layer, or layers, comprises at least one stiffening polymer selected from the group consisting of: low density polyethylene, linear low density polyethylene, high density polyethylene, blends thereof, polypropylene random copolymer, styrene/butadiene copolymer, polystyrene, ethylene-vinyl acetate copolymer and cyclic-olefin copolymer, provided that when more than one inner layer is present, the inner layers can be the same or different. The skin layers, which also may be the same or different, comprise at least one of: low density polyethylene; a blend of low density polyethylene and linear low density polyethylene; a blend of low density polyethylene and very low density polyethylene; polystyrene; ethylene-vinyl acetate copolymer; a blend of ethylene-vinyl acetate copolymer and linear low density polyethylene; cyclic-olefin copolymer; styrene-butadiene copolymer; or, polypropylene random copolymer, provided that the skin layers are devoid of a homogeneously branched polyethylene resin prepared with a single site catalyst. The films are reported to have haze values of less than about 15%.
Plastic films have widely been employed for prepackaging various products including foods, such as vegetables and the like. See, for example, U.S. Pat. No. 6,489,016 which discloses multilayer packaging films of polyolefin. Also disclosing such packaging materials and packages made therefrom are U.S. Pat. Nos. 6,383,582; 5,750,262; 5,783,270; and 5,755,081; and Statutory Invention Registration No. H1727. Such prepackaging films serve for covering and enclosing commercial articles, such as foodstuffs including liquid foodstuffs, as individual articles or in a lot allotted or subdivided into small portions placed on a plastic carrier tray or the like. The success of such packaging has been attributed to the broad use of thermoplastic heat sealing which is capable of providing the highest degree of package integrity, security and durability. Furthermore, it is required for the prepackaging film that the film is transparent and easily cut, together with practical performance attributes such as resistance to fogging on the inner face when packaged, resistance to break-through and so forth.
Optical properties such as haze and transparency are important issues related to packaging films. It is important for the customer to be able to see through packaging so that the product is well-presented. Another salient attribute of packaging films is their coefficient of friction (hereinafter “COF”). For example, a reduction in film-to-film and film-to-metal friction can provide for easier feeding of film sheets into automatic converting and packaging equipment. Films with high COFs (low slip) are difficult to handle and tend to wrinkle leading to high converting losses.
As reported in Cyclic-olefinic Copolymers as Non-Migrating, Polymeric Slip Additives in LDPE and LLDPE Cast Films by David R. Constant of Ticona LLC 2002 PLACE Conference Proceedings (Conference Papers)] there are a large number of options for slip additives including COPs, but that all slip additives “seem to come with compromises, however, generally in film clarity or cost”. It is further mentioned that although COPs have a significant effect on COF. of the films, that effect is also associated with an increase in surface haze.
An object of the present invention is to provide a polyethylene composition and film for packaging applications that have low COFs (static and dynamic) as well as superior optical properties. It is a further object of the present invention to provide a method for making a polyethylene film having surfaces with low COFs and haze values.